What can I use my event rewards for?

Every time I finish an event, I get a popup with a bunch of numbers in there. I think it's money, EXP, Karma and renown? Anyway, I get what EXP is for, and money is also pretty clear. however, what are the other 2 for? should I save them? for what? do they decay? is there a cap? anything else important?

The renown thing is an extra bar that you get only for doing stuff related to a nearby renown heart or "heart quest" as they're usually called. It's just a progress tracker showing you how many more tasks you have to do to complete the "quest". These do not exist in high level zones and in lower zones they're only really there to guide you to areas of importance. You get a small amount of money and karma and some XP for completing them.

Normal events give money, experience and karma. Karma is used for buying certain stuff. It's like a currency, some stuff costs karma instead of money. And no, karma doesn't decay.